This invention relates to electric heaters, and more particularly, it relates to electric heating elements and heaters suitable for use in molten metals such as molten aluminum, for example.
In the prior alt, electric heaters used for molten aluminum are usually enclosed in ceramic tubes. Such electric heaters are very expensive and are very inefficient in transferring heat to the melt because of the air gap between the heater and the tube. Also, such electric heaters have very low thermal conductivity values that are characteristic of ceramic materials. In addition, the ceramic tube is fragile and subject to cracking. Further, heaters are limited by the ability of the heating element to withstand heat. Thus, there is a great need for an improved electric heater suitable for use with molten metal, e.g., molten aluminum, which has an improved heating element and which is efficient in transferring heat to the melt.
Aluminum is frequently delivered to customers in molten form. The benefits are substantial energy savings and product availability in a ready-for-use (molten) condition. Trailer mounted transport crucibles are used for this purpose. Since the heat loss from these crucibles is high, transport time is limited to a few hours, and considerable superheat must be added to the metal to ensure delivery at minimum acceptable temperature. It is common practice to heat molten aluminum to temperatures above 1700.degree. F. for the purpose of adding sufficient superheat. Direct impingement gas fired burners are used for this purpose.
High temperature is undesirable because the resulting increase in metal oxidation rate generates skim. Melt loss can exceed 10%. Further, metal quality rapidly deteriorates since hydrogen solubility in aluminum is an exponential function of temperature, and oxides are formed. Refractory life is reduced by high temperature, and wall accretions build up and limit crucible metal capacity. Finally, the hazards associated with handling molten aluminum increase significantly with elevated temperature. The present invention minimizes these costs and greatly extends the delivery range by providing an improved crucible for molten metal.